Each benzene and para-xylene are industrially important starting materials. Benzene is for instance widely used as a feed to make cyclohexane which in turn can be used to make nylons, whereas para-xylene is used for the production of for example polyester fibers.
Para-xylene is derivable from a xylene mixture which in addition contains the isomers meta-xylene and ortho-xylene. Of the xylene isomers para-xylene is the most in demand, whereas ortho-xylene and meta-xylene are much less in demand. Thus, it is industrially important to convert ortho-xylene and meta-xylene into para-xylene.
Xylene mixtures are usually obtained by subjecting naphtha to a reforming process which is followed by aromatic extraction. The xylene mixtures can also contain cracked xylenes which have been obtained by subjecting naphtha to a thermal decomposition treatment, followed by aromatic extraction. In addition to the xylene isomers such xylene mixtures also contain high concentrations of ethylbenzene and non-aromatic hydrocarbons such as cyclic aliphatics.
Much research and development work has been directed to processes and catalysts to convert such xylene mixtures into mixtures that contain a high amount of benzene, as well as a high amount of para-xylene. In such conversion processes dealkylation of ethylbenzene mainly into benzene is established, and at the same time isomerization of ortho-xylene and/or meta-xylene into para-xylene.
A difficulty that arises in such xylene mixture conversion processes is the presence of so-called benzene co-boilers. Benzene co-boilers are non-aromatic hydrocarbon impurities such as cyclohexane and methylcyclopentane that are very difficult to separate from benzene by distillation because they have a boiling point close to that of benzene. Since benzene as a starting material needs to meet stringent purity standards these benzene co-boilers need to be removed by means of an additional extraction step which is expensive and time-consuming.
Another difficulty is that individual isomers of xylene and ethylbenzene are close in their boiling point, making it very difficult to separate para-xylene from ethylbenzene by distillation.
Alkylaromatic conversion catalysts have been described in WO0038834 and WO2009016134.